Distance measuring devices for motor vehicles which use ultrasonic signals to determine the distance to obstacles in the surroundings of the vehicle are known. An ultrasonic signal is emitted by a sensor, and the signal is reflected by an obstacle. The reflected signal is received again by the sensor that emitted it, or possibly also by another sensor. The distance of the obstacle to the vehicle is determined from the propagation time of the signal. Ultrasonic distance measuring devices of this type are used mostly for supporting the driver in parking the vehicle. These devices make use of the fact that the entire surroundings of the vehicle, but at least a wider angular range in front of and/or behind the vehicle, may be covered using relatively few sensors. In general, however, the range of ultrasonic sensors is limited. For other applications, for example, for adaptive cruise control (ACC), radar sensors are used, in which radar signals are emitted, reflected by an obstacle, and received again by the radar sensor. The distance to the obstacle is determined from the signal change due to the propagation time. Radar sensors of this type are characterized in that they have a relatively long range of up to 200 m. On the other hand, for monitoring traffic, the monitored angular range should not be so great that obstacles on the sidewalk or on a parallel street are perceived as a vehicle traveling ahead. To implement the different applications such as parking assistance and adaptive cruise control in a vehicle, providing both ultrasonic sensors and radar sensors for the different applications in a vehicle is known.
Furthermore, performing a self-test for the particular systems is also known. In particular in the case of ultrasonic sensors, contamination may also result in a possibly gradual drop in performance. If this drop in performance is not detected, the system may go entirely blind. If the ultrasonic sensors belonging to a system are contaminated to different degrees, a blindness detection procedure known from German Patent Application No. DE 199 24 755 may be performed by analyzing the direct crosstalk of sensors. Here it is checked whether, when a signal is emitted which is transmitted to another sensor via crosstalk by the bumper, a reflected signal from an obstacle is also received. However, if all sensors are contaminated in the same way, a drop in performance may possibly not be detected.